Memory Mapping Methods in the Super Nintendo

Not only is the Super Nintendo an all-around great platform, both during its prime in the 90s and now during the nostalgia craze, but its relative simplicity compared to modern systems makes it a lot more accessible from a computer science point-of-view. That means that we can get some in-depth discussion on how the Super Nintendo actually does what it does, and understand most of it, like this video from [Retro Game Mechanics Explained] which goes into an incredible amount of detail on the mechanics of the SNES’s memory system.

Two of the interesting memory systems the SNES uses are called DMA and HDMA. DMA stands for direct memory access, and is a way for the Super Nintendo to access memory independently of the CPU. The advantages to this are that it’s incredibly fast compared to more typical methods of accessing memory. This isn’t particulalry unique, but the HDMA system is. It allows the SNES to do all kinds of interesting tricks with its video output display like changing color gradients and doing all kinds of masking effects.

If you’re interested in the inner workings of classic consoles like the SNES, this video gets way down in the weeds in the system itself. It’s interesting to see how programmers were able to squeeze more capability from these limited (by modern standards) systems by manipulating memory like the DMA and HDMA systems do.  [Retro Game Mechanics Explained] is a great resource for exploring in-depth aspects of lots of classic games, like how speedrunners can execute arbitrary code in old Mario games.

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34C3: Roll Your Own Network Driver In Four Simple Steps

Writing your own drivers is a special discipline. Drivers on the one hand work closely with external hardware and at the same time are deeply ingrained into the operating system. That’s two kinds of specialization in one problem. In recent years a lot of dedicated networking hardware is being replaced by software. [Paul Emmerich] is a researcher who works on improving the performance of these systems.

Making software act like network hardware requires drivers that can swiftly handle a lot of small packets, something that the standard APIs where not designed for. In his talk at this year’s Chaos Commnication Congress [Paul] dissects the different approaches to writing this special flavor of drivers and explains the shortcomings of each.

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Better Stepping With 8-Bit Micros

The electronics for motion control systems, routers, and 3D printers are split into two camps. The first is 8-bit microcontrollers, usually AVRs, and are regarded as being slower and incapable of cool acceleration features. The second camp consists of 32-bit microcontrollers, and these are able to drive a lot of steppers very quickly and very smoothly. While 32-bit micros are obviously the future, there are a few very clever people squeezing the last drops out of 8-bit platforms. That’s what the Buildbotics team did with their ATxmega chip — they’re using a clever application of DMA as counters to drive steppers.

The usual way of driving steppers quickly with an ATMega or other 8-bit microcontroller is abusing the hardware timers. It’s quick, but there is a downside. It takes time for these timers to start and stop, and if you’re doing it two hundred times per second with four stepper motors, that clock jitter will ruin your CNC machine. The solution is to use a DMA channel to count down, with each count sending out a pulse to a stepper. It’s a clever abuse of the hardware, and the only drawback is the micro can’t send more than 2¹⁶ pulses per any 5ms period. That’s not really an issue because that would mean some very, very fast acceleration.

The Buildbotics team currently has a Kickstarter running for their four-axis CNC controller using this technique. It’s designed for Taig mills, 6040 routers, K40 lasers, and other various homebrew robots. It’s an interesting solution to the apparent end of the of the age of 8-bit microcontrollers in CNC machines and certainly worth checking out.

Better LEDs through DMA

While regular Hackaday readers already know how to blink a LED with a microcontroller and have moved onto slightly more challenging projects such as solving the Navier-Stokes equations in 6502 assembly, that doesn’t mean there’s not space for newbies. [Rik] has published a great tutorial on abusing DMA for blinkier glowy things. Why would anyone want to learn about DMA techniques? For blinkier glowy things, of course.

This tutorial assumes knowledge of LED multiplexing and LED matrices, or basically a bunch of LEDs connected together on an XY grid. The naive way to drive an 8×8 grid of LEDs is attaching eight cathodes to GPIO pins on a microcontroller, attaching the eight anodes to another set of GPIO pins, and sourcing and sinking current as required. The pin count can be reduced with shift registers, and LED dimming can be implemented with PWM. This concludes our intensive eight-week Arduino course.

Thanks to microcontrollers that aren’t trapped in the 1980s, new techniques can be used to drive these LED matrices. Most of the more powerful ARM microcontrollers come with DMA, a peripheral for direct memory access. Instead of having the CPU do all the work, the DMA controller can simply shuffle around bits between memory and pins. This means blinker projects and glowier LEDs.

[Rik]’s method for DMAing LEDs includes setting up a big ‘ol array in the code, correctly initializing the DMA peripheral, and wiring up the LED matrix to a few of the pins. This technique can be expanded to animations with 64 levels of brightness, something that would take an incredible amount of processing power (for a microcontroller, at least) if it weren’t for the DMA controller.

The setup used in these experiments is an STM32F103 Nucleo board along with the OpenSTM32 IDE. [Rik] has released all the code over on GitHub, and you are, of course, encouraged to play around.

Understanding DMA

In the world of computers, the central processing unit (CPU) is–well–central. Your first computer course probably explained it like the brain of the computer. However, sometimes you can overload that brain and CPU designers are always trying to improve both speed and throughput using a variety of techniques. One of those methods is DMA or direct memory access.

As the name implies, DMA is the ability for an I/O device to transfer data directly to or from memory. In some cases, it might actually transfer data to another device, but not all DMA systems support that. Sounds simple, but the devil is in the details. There’s a lot of information in this introduction to DMA by [Andrei Chichak]. It covers different types of DMA and the tradeoffs involved in each one.

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Driving 16 WS2812B Strips with GPIOs and DMA

[Martin Hubáček] wrote in with his WS2812 LED library for the STM32F3 series processors. [Martin]’s library takes the same approach as [Paul Stoffregen]’s OctoWS2811 for the Teensy, and [Erich Styger]’s for the Freescale FRDM-K64F board. That is, it uses three DMA channels to get the signal out as fast as possible.

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PIC32 DMA SPI

[Mike] wanted to drive several SPI peripheral from a PIC32. He shows how much latency his conventional interrupt handlers were taking away from his main task. He needed something more efficient. So he created the SPI channels using DMA. He also made a video (see below) with a very clear explanation about why he did it and shows oscilloscope traces about how it all works.

Although the project is specific to the PIC32, the discussion about DMA applies to any computer with direct memory access. The only thing missing is the code. However, there are plenty of examples on the web you can look at, including a Microchip webinar.

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